This invention relates generally to RF power transistors and, more particularly, the invention relates to a repetitive feed structure for such devices which reduces thermal gradients.
FIG. 1 is a schematic of a bipolar NPN transistor having emitter and collector regions separated by a base region. Applying Kirchoff's current law, emitter current, I.sub.e, equals the sum of base current, I.sub.b, and collector current, I.sub.c,
Typically, emitter and base contacts are provided on one (front) surface of the transistor and the collector contact is made to the opposing (back) surface with base and emitter wire bonding pads positioned centrally in the metal structure with current flowing to and from interdigitated finger contacts as shown in FIG. 2. In a typical RF power transistor cell layout, repeating techniques are used to replicate a base and emitter bus structure across the die area with the bus structure arranged in rows of alternating or "flipped" emitter-base orientation. This causes alternating interdigitated base and emitter currents to flow in the same direction as the collector current or in opposite direction to the collector current.
The bipolar transistor current relationships introduce mutual magnetic coupling or magnetic flux linkage between currents. As illustrated in FIG. 3, parallel currents flowing in the same direction produce negative magnetic coupling with the current tending to oppose each other. However, when the parallel currents flow in opposite directions as shown in FIG. 4, mutual coupling is enhanced or positive and the currents tend to support each other.
If the conductors in FIG. 4 are reoriented such that the angle between two current flows is 90.degree., coupling is minimal. It can be shown that the mutual inductance arising from two coupled wires is given by: ##EQU1## where s1 and s2 are vector elements along the direction of the wires, r12 is the distance between these elements, L1 and L2 are the line integral paths, and the permeability .mu. is constant. Clearly, when s1 and s2 are orthogonal, their dot product is zero, and the resulting mutual inductance is also zero. Accordingly, 90.degree. coupling can be beneficial.